Unwanted child bearing

by Maxwell Addo
(Ghana)

We are legitimately married and have four children.We are not ready for a child again.My wife missed he period and want to see a medical Doctor to let it pass.After giving birth to the fourth child,doctor adviced her not tro bring forth a child a gain due to her health.Is this a sin?As we are not financially strong to cater for another child as even it is hard caring for the four..Is this a sin seeing the medical Dr.making it to pass?